This invention relates to methods for measuring particulate matter in a gas, such as for environmental sampling.
Epidemiological studies in the U.S.A. and abroad have shown associations between mortality and morbidity and human exposures to ambient particulate matter (Schwartz and Dockery, Am. Rev. Resp. Dis. 145:600-604 1992; Pope et al., Am. Rev. Res. Dis. 144:668, 1992). To date, there is limited knowledge about the physical or chemical property of particulate matter that are responsible for these health effects. Whether the particulate matter is a surrogate for an unknown toxicant or is the responsible agent itself for the observed health effects, there is an increasing interest in developing accurate measurements in the near future. Currently, the agencies are receiving the permitted ranges for ambient particles. Although no decision has been made about the form of the new standard regarding the size cutoff and concentration level parameters, there is a mandate for the use of a reference or equivalent method.
The majority of current particle mass measurement methods use a size selective inlet to remove particles above a certain size, usually 10 .mu.m in diameter (PM.sub.10). There is a large discrepancy between the different methods. This is due to the effects of wind velocity on the inlet performance. These effects are more pronounced for coarse particles, those with aerodynamic diameter between 2.5 and 10 .mu.m. Thus, if the revised standard decreases the particle size, this problem will be minimized. Most of the available data on PM.sub.10 and PM.sub.2.5 have been obtained using gravimetric methods. The collected particles, usually on Teflon filters, are weighed using microbalances under constant specified temperature and relative humidity conditions. Gravimetric methods are not sensitive enough to measure samples within less than 24 hours. In addition, attempting to obtain a better time resolution for ambient particle concentrations routinely for large monitoring networks is presently cost-prohibitive and impractical. Therefore, development of continuous particle monitors will be important in order to establish comprehensive monitoring networks that will provide information on temporal and spatial variability of particle mass concentration in a cost-effective way.
To date, there are several direct-reading instruments; an excellent review of such instruments has been given by Swift, Air Sampling Instruments, edited by S. V. Hering. American Conference of Governmental Industrial Hygienists, Inc., Cincinnati, Ohio (1989). Nevertheless, there are a number of questions regarding the quality of data these instruments can provide. Some methods use the optical properties of ambient particles to measure their mass (e.g., the mini-RAM instrument, nephelometers, instruments measuring the Coefficient of Haze, etc.). However, because the chemical composition and size of particles varies significantly with time and space, it is difficult to establish a constant relationship between particle mass and extinction coefficient. These methods may be of limited use, i.e. for studies of fine aerosols in locations where the composition of particles is not expected to vary significantly. An advantage in some of these methods is that collection of particles on a filter media may not be required. Interactions between particles on filter media and/or particle to gas conversions and vice-versa can result in significant overestimation or underestimation of particulate mass. This problem applies to both continuous and integrated methods that collect particles on filter media.
Another continuous particle mass method is the beta-gauge (Macias and Husar, in Fine Particles, edited by B. Y. H. Liu, Academic Press, New York, 1976). The attenuation of the beta rays passing continuously through the filter is proportional to the particle mass collected on the filter. However, the relationship between mass and energy absorption depends to some extent on the particle composition. In addition, a radioactive material is required as an energy source. Therefore, the beta-gauge method may not be the method of choice for the future.
Quartz crystal piezobalances measure particulate mass directly through particle impaction on an oscillating quartz surface (Lundgren et al., in Fine Particles, edited by B. Y. H. Liu, Academic Press, New York, (1976)). More specifically, a disk of quartz oscillates in an electric circuit at a highly stable resonant frequency which decreases in direct proportion to the particulate mass impacting and adhering onto the sensor. Although such instruments have been used with some success in providing direct readings of aerosol mass concentrations, they suffer from several potential shortcomings. First, the high oscillating frequency of the quartz crystal can lead to saturation effects. Second, the fact that particle collection is done by impaction leads to collection uncertainties. Several investigators (Daley and Lundgren, "The performance of piezoelectric crystal sensors used to determine aerosol mass concentration." Am Ind. Hyg. Assoc. J. 36:518, (1975); Lundgren Am. Ind. Hyg. Assoc. J. 38:580-588, (1977)) found that the frequency change for a given incremental mass deposit on the sensor does not remain constant as the sensor becomes loaded. This is due to the change in the particle collection patterns over time. Some aerosols, such as CaCO.sub.3 deposit uniformly in the beginning, but as the loading increases, the freshly depositing particles tend to deposit near the center of the sensor, probably because of the change in the electrical conductivity of the collection surface. Other types of aerosols, such as Fe.sub.2 O.sub.3, while they initially deposit uniformly over the sensor, as loading increases, the impacting particles do not adhere as well to the surface. As a result, the frequency of a loaded crystal does not change as much as that of a clean crystal for the same increment of mass, thus the loaded crystal senses a lower concentration than the actual. Finally, aerosols consisting of carbonaceous particles which are composed of long stable chains of very small primary particles cannot be measured with piezobalances; the chain aggregates contact the sensor at 2 to 3 points with most of the particulate mass waving above the sensor surface. This observation was confirmed with experiments using black carbon particles as the test aerosol to be collected on the piezobalance. The response of the instrument became non-linear within few minutes after the beginning of the experiments.
The Tapered Element Oscillating Microbalance (TEOM.RTM.) is a recently developed method that originally appeared to be very promising (Patashnick and Rupprecht, "Continuous PM-10 Measurements Using the Tapered Element Oscillating Microbalance" J. Air Waste Manage. Assoc. 41:1079-1083 (1991)). According to this method, the air sample is heated up to 50.degree. C. to remove moisture, and particles are subsequently collected on a TEFLON.RTM. filter that oscillates at the top of a metal rod. The amplitude of the oscillation decreases as the mass of the particles collected on the filter increases. Although this method is highly sensitive, its measurements are subject to a number of interferences; significant losses occur for semivolatile organic and inorganic compounds that in some areas can represent relatively large fraction of the total particulate matter. This problem is more pronounced for PM.sub.2.5, which includes unstable compounds such as ammonium nitrate and carbonaceous aerosols. For areas such as California and large urban environments, this method would significantly underestimate particle mass concentrations. Also, as the composition of the air sample changes, the partitioning of air pollutants between the gas and particle phase changes, therefore adsorption and/or desorption processes can take place on the filter (depending on whether the air sample becomes more or less polluted). Due to the sensitivity of the method, these phenomena can cause either negative or positive artifacts. The gains and losses of mass on the filter are a serious problem, not just of the TEOM.RTM., but of any method that collects particles on a filter over a prolonged period of time (on the order of days). In the case of the TEOM.RTM., the filter media are usually exposed for a week. Finally, this method presents oscillations in its response which cancel out if a large number of measurements are added to determine a multi-hour concentration estimate; however, over shorter time intervals the measurement errors due to this oscillation can exceed 20-30%.
Other direct-reading methods to measure particle concentration include optical and electrical counters. Optical counters make use of the interaction between light and particles. A review on the theory of optical aerosol behavior and its application to particle measurement is discussed by Willeke and Liu, "Single Particle Optical Counters: Principle and Applications" Fine Particles, edited by B. Y. H. Liu, Academic Press, New York (1976). Most of the optical systems count light pulses scattered from particles that flow, one by one, through an intensely illuminated zone. Some of their limitations are low sampling flow rates, and the fact that the smallest detectable particle size is about 0.3 .mu.m. Since there may be a significant fraction of the fine ambient particulate mass associated with particles smaller than 0.3 .mu.m, optical counters are not adequate to measure the entire atmospheric particle range.
Electrical counters are based on charging the sampled aerosols and measuring the ability of particles to traverse an electrical field. Most of these counters draw particles through a cloud of either unipolarly or bipolarly charged ions, and each of the particles acquire a quantity of charge that is simply related to its size. Subsequently, the particles are drawn into a radially symmetric electrical field where particles smaller than a certain size, which depends on the intensity of the field, are collected onto the walls of the collecting device. By changing the field voltage, the particle size distribution can be obtained (Liu and Pui, "Unipolar Charging of Aerosol Particles in the Continuum Regime", J. Aerosol Sci., 6:249 (1975); Whitby, "Electrical Measurement of Aerosols", Fine Particles, edited by B. Y. H. Liu, Academic Press, New York, (1976)). The most widely used instrument of this category is the Differential Mobility Analyzer manufactured by TSI Inc. (St. Paul, Minn.). This technology has been proven to be useful in generating monodisperse aerosol in the size range 0.01-1.0 .mu.m. However, particles larger than about 1.0 .mu.m in diameter cannot be accurately measured by this instrument because the relationship between particle charge and particle size is not monotonic for particles larger than 1.0 .mu.m.
Using the Differential Mobility Analyzer in conjunction with an optical counter would make it possible to measure a broad size range of atmospheric particles. Nevertheless, even in this case, the combined optical/electrical counter may still suffer from two shortcomings. The first shortcoming arises from the fact that these counters measure the number distribution of particles which they subsequently convert to volume distribution. To measure the mass distribution one needs to know the particle density which is known to vary depending on the sampling location and the sampling period (e.g., winter vs. summer). The second shortcoming is intrinsic to converting a number to a volume distribution. The number distribution of ambient particles is dominated by ultrafine particles, in the size range 0.01-0.1 .mu.m. The coarser the particles, the smaller their number concentration becomes. However, when converting a number to volume distribution, a 1.0 .mu.m particle is weighed as much as 10.sup.3 0.1 .mu.m particles and 10.sup.6 0.01 .mu.m particles. Consequently, this conversion is bound to lead to counting errors because a low background concentration of coarse particles (which may just be within the noise of the instrument) will be converted to a significant fraction of the volume distribution.
To conclude, there is a great need for the development of a continuous particle mass measuring instrument. As long as the health effects studies cannot identify the chemical constituents or physical properties of particulate matter that are responsible for the observed morbidity and mortality, we ought to include in particle measurement all individual components. Continuous mass measuring methods should be robust for both stable species (e.g., sulfates and dust) as well as unstable species (e.g., ammonium nitrate, chloride, and SVOCs). Furthermore, the effects of relative humidity and temperature on particle mass measurement should be addressed.
As discussed below, particles should preferably be conditioned to 40% RH (when ambient RH exceeds 40%) to minimize the interference due to water vapor. The air sample should preferably not be heated because unstable compounds can easily volatilize resulting in an underestimate of particle mass. In situ aerosol measurements are very desirable since they are free from particle interference and adsorption and desorption problems. However, if use of filter media is necessary, the collection surface should preferably not be used for more than 20-30 minutes in order to avoid filter mass gain or loss problems. The continuous method should either regenerate the collection surface or use a new surface for each measurement.
The purpose of the present invention is to provide a continuous mass measurement method that incorporates all of the features mentioned above. Mass concentration measurement is based on monitoring the pressure drop across a porous membrane filter (or its equivalent) over a period of time. As will be discussed below, this pressure drop is a linear function of the particle mass concentration of the sampled air.